My first thought is Napalm strike. It could function like a minor artillery card but deal damage to infs. in addition to ground units and maybe become a damaging terrain feature for a round or two. The other the other is a stealth fighter card to make a air unit invisible as a permanent. A related buff card could be a radar jamming that could have a 1-3 with limited durations. Maybe have the buff end when the unit attacks or make 2 separate buffs where 1 makes the air unit like a guerrilla inf and the other ends when the unit attacks.